The present invention relates to a method for retrieving subscriber identifications in a communication network having multiple subscribers which are connected to a ring-shaped transmission path. Furthermore, the present invention relates to a ring-bus communication network comprising a configuration subscriber and further subscribers connected to the ring bus.
Present-day concepts of industrial automation, i. e. controlling and monitoring of technical processes by means of software, are based on the idea of a central control system having a distributed sensor/actuator level. Therein, the subscribers communicate with each other and with superordinate systems via local communication networks known as Local Area Networks (LANs). LANs applied in industrial automation are usually configured as what is known as master-slave communication networks in which master subscribers form the control level and slave subscribers form the sensor/actuator level.
A preferred topology in the construction of LANs is the ring topology in which the subscribers are connected to each other via two point connections so that a closed ring is formed. Such ring-bus systems automatically prevent the collision of circulating data packets, are simple to be scaled and easy to program. Furthermore, network communication in ring-bus systems is deterministic and all subscribers have the same access options.
The safe and reliable data transmission is an essential requirement to LANs when used in industrial automation. When LANs are used for controlling machines, it has to be safe-guarded that no danger is posed to humans and the environment in case one machine component fails. Therefore, LANS in industrial automation, in the following also referred to as automation networks, usually work with what is known as the fail-safe principle according to which the automation network is switched to a safe status if essential components fail. Therein, it is the task of automation computers in the automation network, i. e. master subscribers, to process the process signals for carrying out control functions in a current and unadulterated manner when carrying out safety-relevant control functions according to the failsafe principle and to always report a safe process status to the sensors/actuators of the automation network, i. e. the slave subscribers.
For a safe and reliable communication, particularly also in automation networks, it is an essential requirement that the correct slave subscriber is always addressed. Therefore, the slave subscribers are assigned with an identification, usually an address, by means of which they may be identified unambiguously. The subscriber addresses are then at the same time stored in the control programs to be carried out which run on the master subscribers in the automation network. However, setting the subscriber addresses is usually complicated and error-prone. Setting the addresses in the individual subscribers is conventionally carried out by means of address-selecting switches. Then, the set subscriber addresses have to be transmitted to the master subscribers which carry out the control programs. In this context, there is the danger of falsely entering or altering an address, respectively, which presents a high risk, especially in the case of safety-relevant subscribers. Therefore, for ensuring that addresses are entered in a reliable manner, a high organizational effort is required.
In order to facilitate the setting of subscriber addresses, in particular in automation networks, methods for setting addresses by means of software have recently been applied. For example, in DE 10 2008 037 093 A1 a method for automatically addressing subscribers in a communication network is suggested in which the subscribers are by means of a common bus connected to a configuration subscriber which carries out the address allocation by transmitting a setting signal to a first downstream subscriber which thereupon allocates a first address to be allocated to itself. This subscriber then transmits the allocated addresses in the form of a modulated output signal to the next subscriber which then allocates the second address to be allocated to itself. This method is then carried out until the last subscriber on the bus has allocated an address to itself. The danger of this approach, however, is that one subscriber allocates a wrong address to itself due to a program error, this error then propagating into the entire automation network.
DE 37 36 081 A1 discloses a further automatic method for setting addresses for subscribers in a communication network in which the addresses are allocated by a configuration subscriber. The configuration subscriber then again retrieves the addresses from the subscriber in order to verify if the correct address has been set. In this approach, however, there is also the problem that the wrong address is respectively set or reported due to transmission errors.